Image intensifier faceplate

ABSTRACT

A glass faceplate having a central area of required useful diameter surrounded by a narrow ring of opaque glass and an outer glass mounting flange. Light entering the flange or ring is prevented from reaching the central area as stray light or glare in the system and spurious reflections at the interface of the ring and central area are minimized by a refractive index match of the two innermost glasses.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to faceplates for image intensifiers and hasparticular reference to improvements in clear glass cathode plates.

2. Discussion of the Prior Art

One form of image intensifier tube uses a clear glass cathode (input)faceplate which receives image-forming light intended to be intensifiedfor enhanced viewing. Light entering prior art faceplates near theiredges, however, can become reflected from the edges or adjacent portionsas stray light which produces glare in the system and degradation ofemitted images.

With the above in view, an object of the present invention is toeliminate this cause of stray light in simple and straightforwardfashion and without undue cost.

More particularly, the invention has the objective of affording asolution to problems of glare in image-itensifier systems which can beaccomplished simply, efficiently and successfully with ordinary skill inthe art.

Other objects and advantages of the invention will become more readilyapparent from the following description.

SUMMARY OF THE INVENTION

Objects and advantages of the invention are accomplished by provision ofa glass faceplate having a central area of useful diameter surrounded bya narrow ring of opaque (black) glass and an outer glass mountingflange. The structure is adaptable to conventional or special imageintensifier tubes and uniquely prevents light entering its flange orstriking the ring of opaque glass from reaching the central area asstray light or glare in the system. Refractive indices of the centralarea and opaque ring glasses are preferably approximately matched toprevent spurious reflections at their interface and are further closelymatched in expansion coefficients.

Details of the invention will become more readily apparent from thefollowing description when taken in conjunction with the accompanyingdrawings.

IN THE DRAWINGS

FIG. 1 is an illustration in cross-section of an image intensifier tubeincorporating an embodiment of the present invention;

FIG. 2 is an enlarged fragmentary cross-sectional view of a corner ofthe tube of FIG. 1 wherewith details of the improved faceplate areillustrated with greater clarity;

FIG. 3 is a view similar to that of FIG. 2 but which illustratesstructure of the prior art for ease of understanding of the improvementafforded by the present invention;

FIG. 4 illustrates, in perspective, the construction of a preform fromwhich image intensifier faceplates may be made according to theinvention; and

FIG. 5 is a diagrammatic illustration of steps which may be used to workthe preform of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Faceplate 10 of image intensifier tube 12 is illustrated in FIGS. 1 and2 as comprising a clear glass portion 14 of useful diameter (typically18 mm), a thin ring 16 of opaque (light-absorbing) glass having athickness of approximately 1 mm or less and an outer ring or flange 18of compatible glass and an overall diameter typically 30 mm, formounting the faceplate to envelope 19 of tube 12.

Photocathode layer 20, activated by image-forming light passing throughportion 14 of plate 10, emits electrons which are drawn throughelectronmultiplier 22 and received in intensified (multiplied) fashionby phosphor layer 24 for conversion to intensified image-forming light.Electrical potentials V₁,V₂ and V₃ (e.g. 500 V, 1000 V and 5000 Vrespectively) are used in conventional fashion to accelerate thecathode-emitted electrons through the tube. Those interested in detailsof the use of photomultipliers may refer to U.S. Pat. No. 3,979,621.

This invention relates more particularly to improvements inlight-receiving cathode faceplates and is applicable to various otherimage intensifier systems including those employing image-invertingmeans not shown or requiring description herein.

Referring to FIGS. 2 and 3 wherewith improvement of the invention isdemonstrated, it can be seen that prior art cathode plates, e.g. plate10a of FIG. 3, are subject to the production of image glare resultingfrom stray light reaching the photocathode after reflection from edge 26of the plate. For example, image-forming light L entering the faceplateoutside the useful central area may be reflected from the edge 26 andreach photocathode 20a as stray light L₁ causing a veiling glare in theamplified image. An accummulation of similar reflections from edge 26resulting from light rays L₂ seriously degrade images produced andemitted from prior art intensifier tubes employing solid glass cathodes,e.g. of the type illustrated in FIG. 3.

As illustrated in FIGS. 1 and 2, the present invention overcomes theproblems of glare in image intensifier systems using solid glass cathodeplates as follows:

Plate 10 receives image-forming light L through center portion 14 foractivation of photocathode 20 but prevents other light L₂ fromactivating photocathode 20 or otherwise interferring with light L. Forexample, light L₂ entering flange 18 and reflecting from edge 26 becomesabsorbed by opaque glass 16 and prevented from reaching portion 14and/or cathode 20. Also, light directly striking glass 16 becomessimilarly absorbed. Additionally, with indices of refraction of glasses14 and 16 substantially matching, spurious reflections 28 at interface30 are avoided, i.e., light L₃ entering plate 10 along a direct path andstriking ring 16 will become absorbed by ring 16.

From the foregoing it should be apparent that the present inventionovercomes problems of image degradation in intensifier systems usingclear glass plates of the prior art configuration as cathode inputfaces.

A preferred method of making the present glass cathode plate isillustrated in FIGS. 4 and 5.

A glass preform 30 (FIG. 4) may be constructed of a surface ground andpolished rod 32 of central glass which, for reasons of photocathode 20compatibility, is typically a borosilicate glass. Suitable glasses ofthis type are readily obtainable from glass suppliers.

Ring 16 glass comprises strips 34 tightly assembled around rod 32. Theglass of these strips may be of the type disclosed in U.S. Pat. No.3,901,718 wherein various exemplary compositions are disclosed. Othersuitable compositions may, of course, be used provided the central andstrip glasses 32, 34 permit fusion and drawing, i.e. have compatiblemelting points and expansion coefficients.

Supporting the assembly of strips 34 and rod 32 is one or more tubes 36of flange 18 glass. Compatibility for fusing and drawing with the formerglasses is required. A borosilicate glass is preferred.

Once assembled, preform 30 is heated, e.g. zonally as depicted in FIG. 5with heating elements 38, and drawn (stretched) to the desiredcross-sectional size of faceplate 10. Fiber drawing techniques may beused for this purpose, details of which may be found in U.S. Pat. Nos.3,037,241; 3,455,667 and 3,278,283.

The boule 40 thus formed (FIG. 5) is annealed and sliced into plates 42which may be machined into desired shape, e.g. that illustrated in FIGS.1 and 2. Cathode 20 is applied and flange 18 is sealed to envelope 44preferably with a suitable soft metallic ring 46.

Those skilled in the art will appreciate that various modifications ofthe present invention may be made to suit particular requirementswithout departure from the spirit of the invention. Accordingly, thepresent illustrations are not to be interpreted as restrictive beyondthat necessitated by the following claims.

I claim:
 1. A faceplate for an image intensifier comprising the fusedassembly of:a central glass plate having a finished edge; a ring oflight-absorbing glass extending about said edge of said central plate;and an outer annular supporting flange of glass.
 2. A faceplateaccording to claim 1 including a photocathode coating over one side ofsaid central glass plate.
 3. A faceplate according to claim 1 whereinsaid central glass plate has a diameter of approximately 18 mm and saidring has a thickness no greater than approximately 1 mm.
 4. A faceplateaccording to claim 1 wherein the refractive index of the glass of saidring is approximately equal to that of said central plate.